Sediment Cover Modulates Lithologic Signals in Mountain Rivers: Implications for Channel Profile Interpretation

Disentangling tectonic, climatic, and lithologic controls on bedrock river profiles is a central challenge in geomorphology. Landscapes with layered rocks represent an end-member case of lithologic complexity where lithology varies through both time and space, as vertical incision causes contacts between layers to migrate. Recent studies using the Stream Power Model (SPM) have highlighted the complex variations in erosion rates that arise in these landscapes, which cannot reach a typical topography steady state where erosion rates are equal everywhere as long as lithology continues to vary. Instead, these landscapes reach a dynamic steady state where erosion rates in individual layers adjust to achieve a landscape-averaged erosion rate that is balanced with uplift. However, the SPM assumes detachment-limited behavior, omitting the role of sediment. Lithology controls bedrock rivers not only by setting bedrock erodibility, but also by producing sediment that is transported and deposited downstream. Sediment deposited on the channel bed can form a protective layer that inhibits bedrock erosion, known as the cover effect. 

In this study, we use the Stream Power with Alluvium Conservation and Entrainment (SPACE) model to quantify how sediment cover effects influence patterns of channel steepness and erosion in horizontally layered rocks. We run a series of simulations modeling landscape evolution through alternating layers of hard and soft rock over million-year timescales, varying the amount of sediment that accumulates on the channel bed across model runs. As sediment cover increases, lithologic knickpoints formed by the contacts between layers become less prominent in the topography, and the effective erodibility contrast between the two rock types is substantially reduced. Additionally, sediment cover effects increase topographic relief and landscape adjustment time compared to the SPM model. Our results demonstrate that identical underlying lithologic configurations produce remarkably distinct channel profiles depending on the degree of sediment cover. This work highlights the importance of considering sediment cover effects when analyzing river profiles, particularly in settings with variable lithology.